Question

I have a number of LINQ classes that inherit from the same base class. Each Linq class has its own table, and then the base class is a sort of link to another table. I’ll try and illustrate the relationship:

//Base Class
abstract class Element<T> 
{
   public string Name {get; set;}
   public string Species {get; set;}
}

public partial class DogElement : Element<DogElement>
{
   public int Legs {get; set;}
   public bool hasHair {get; set;}
}

public partial class CatElement : Element<CatElement>
{
   public bool hasWiskers {get; set}
}

So DogElement and CatElement are separate physical tables in the database. The Species class will read from the Species table. I basically want to have a generic function in Element that groups the T table with the Species table and returns the result as a T after having set .Species.

//Assume T is DogElement for this example
public IQueryable<T> Collection
{
   using (DataBase db = new DataBase())
   {
     var k = from t in db.GetTable<T>()
             where t.SomeID == SomeOtherID
             select t;

     k = { {Legs = 4, HasHair = True, Species = <null>, Name = <null>}, ...}

     I basically want a query that will return an IQueryable<DogElement> with Name
     and Species set from a join query.  What is the best way to set the Name and 
     Species values?  

   return k;
} 

Answer 1

Essentially you are asking SQL to handle inheritance (it doesn’t do that)… and LINQ doesn’t model inheritance either. A good object relational mapper (ORM) will help you piece something together more generically.

That said, here’s a quick discussion.

First off, I think it is better to write one LINQ query for each species-animal pair.

You could write a union, switch in the LINQ select, create a new animal, and then cast that animal to a species in one go… then outside the LINQ statement, recast it to T on the way out. This kind of statement sounds messy though.

Although this is less efficient than one big union statement, have you considered this more maintainable option?

var a from speciies select ..

if(T is Dog){
  additional linq statement}
else if(T is Cat){
  additional linq statement}

that’s two trips to the database (slower), but it’s more maintainable, and often it is better not to over optimise your code on the first pass.